Cardiac pacing system include a cardiac pacer connected to a lead which conducts stimuli from the pacer to an electrode implanted either on the surface of or within the cardiac chambers. Certain cardiac pacers, including those designated VOO pacers, emit stimuli at a certain predetermined rate, in pulses per minute. However, there are certain other models, including those designated VVI pacers, in which the output pulse is suppressed or ineffective in the presence of naturally occurring cardiac electrical activity. Such models detect cardiac electrical activity via the lead during a part of or a portion of the interval termed the alert period between output pulses. When such activity is detected or is of sufficient magnitude to be detected during the appropriate portion of the pulse to pulse interval, a new timing cycle is initiated, with time zero of the new interval being the time of detection of the electrical activity. Such electrical activity detected in, e.g., the ventricle, may be that which normally occurs from the atrium to the ventricle or may be a premature ventricular or atrial contraction dependent upon the lead placement and pacer type.
Such cardiac pacers which detect electrical activity during the alert period must have a sensitivity enabling such detection. The sensitivity of a cardiac pacer is generally considered to be that level, in millivolts, which the electrical activity resulting from the depolarization of the cardiac muscle must exceed within a given time period for the cardiac depolarization to be detected by the sensing amplifier of the cardiac pacer. For example, if the sensitivity of a cardiac pacer has been set at one millivolt, then the cardiac electrical depolarization must obtain the one millivolt level within, for example, a 30 milliseconds time period to be detected by the cardiac pacer, thereby suppressing or rendering ineffective the output stimulus and resetting the timing cycle of the cardiac pacer.
The sensitivity of these "sensing" cardiac pacers may be preset during manufacture. In other models of sensing cardiac pacers, the sensitivity may be externally programmed to various levels. Such externally programmed changes in sensitivity are accomplished by the use of an external device and usually require a visit to the physician's office. Therefore, there is no automatic change in sensitivity levels if spontaneous electrical activity is not detected by the implanted cardiac pacer. Also, there is no change in sensitivity in those situations in which the sensitivity is below that needed to detect the electrical activity of a spontaneous depolarization.
The importance of proper sensitivity adjustment cannot be overemphasized. If the sense amplifier of the pacer is not sensitive enough, proper sensing will not occur. On the other hand, if the sense amplifier is too sensitive, it may sense noise and myopotentials. Although a physician may be able to maintain proper sensitivity of the pacer, many patients are unable to see their physicians often. Thus although the sense amplifier of a pacer is programmed correctly at a particular time, the parameters may change at a subsequent period of time and the sensitivity of the sense amplifier may be either too low or too high.
Therefore, it is an object of the present invention to provide a cardiac pacer in which sensitivity is automatically adjusted to sense spontaneous cardiac electrical activity.
Other objects and advantages of the present invention will become apparent as the description proceeds.